1. Field of the Invention
The present invention relates to a ratchet screwdriver, more particularly to a ratchet screwdriver which has a relatively simple structure and which permits operation and replacement of tool bits mounted on two opposite ends of the drive shaft of the ratchet screwdriver.
2. Background of the Invention
FIG. 1 illustrates a first conventional ratchet screwdriver which has a hollow handle 10 formed with an axial receiving chamber 11 with open front and rear ends. An inner coupling sleeve 12 is received in the axial receiving chamber 11 adjacent to the rear end, while an outer coupling sleeve 13 is extended into the axial receiving chamber 11 and is disposed at the front end. The inner coupling sleeve 12 has a first section confining a rectangular hole portion 122 with a rectangular cross-section, and a second section confining a hexagonal hole portion 121 which has a hexagonal cross-section and which is connected to the rectangular hole portion 122. The outer coupling sleeve 13 confines an axial hole 131. A ratchet assembly 15 is mounted on the rear end of the handle 10, and has one end provided with a drive projection 151 which has a rectangular cross-section and which engages the rectangular hole portion 122, and the other end provided with a turning wheel 152 which extends out of the handle 10. A drive shaft 16 is extended through the axial hole 131 and into the axial receiving chamber 11 of the handle 10, and is retained in the axial hole 131 by an annular spring 132 provided on an inner surface of the outer coupling sleeve 13. The drive shaft 16 has two opposite ends, each of which engages a tool bit 161 that has a shank portion with a hexagonal cross-section. The shank portion of one of the tool bits 161 extends into the hexagonal hole portion 121 in the inner coupling sleeve 12, and engages non-rotatably the inner coupling sleeve 12. When the handle 10 is rotated in a certain direction, since the drive projection 151 engages the inner coupling sleeve 12, the inner coupling sleeve 12 engages one of the tool bits 161, and said one of the tool bits 161 engages the drive shaft 16, the drive shaft 16 is rotated axially which in turn cause rotation of another one the tool bits 161 in order to operate a workpiece, such as a screw. However, when the screwdriver applies a forward driving force along the drive shaft 16 during rotation, it is likely that the drive shaft 16 pushes the inner coupling sleeve 12 rearwardly so as to push the ratchet assembly 15 outwardly of the handle 10. In this situation, the inner coupling sleeve 12 and one of the tool bits 161 might be undesirably exposed to injure the user.
Referring to FIGS. 2 and 3, another conventional ratchet screwdriver is shown to include a handle 20 having a head portion 21 which is formed with an axial hole 217 with an open end, and a slot 211. An actuator 221 and a pair of pawl plates 222 are disposed in the slot 211. A tubular housing 23 is sleeved around the headportion 21, and is formed with a circumferentially extending slot 231 to permit extension of an operating protrusion 223 of the actuator 221 therethrough. A drive shaft 24 is extended into the axial hole 217 in the head portion 21. The drive shaft 24 has two opposite ends formed with an axial blind hole 243 for engaging a tool bit 25, and an intermediate portion formed as a ratchet wheel 241. A pair of annular retaining grooves 242 are formed on opposite sides of the ratchet wheel 241 for engaging ball members 215 which are mounted on the head portion 21 and which are biased radially and inwardly by means of springs 216 to engage the annular groove 242, thereby retaining the drive shaft 24 on the head portion 21. As shown, the actuator 221 is operable to move in a direction transverse to an axis of the drive shaft 24 to engage a selected one of the pawl plates 222 with the ratchet wheel 241 such that the drive shaft 24 can be driven by rotating the handle 20 in a certain direction.
However, this type of ratchet screwdriver suffers from the following drawbacks: when the drive shaft 24 is inserted into the axial hole 217 of the handle 20, the pawl plates 222 will be pushed by an end wall of the drive shaft 24. In case the drive shaft 24 is frequently inserted into and removed from the handle 20 for replacement of the tool bits 25, the pawl plates 222 are susceptible to deformation and displacement, thereby affecting adversely the engagement between the pawl plates 222 and the ratchet wheel 241.